Aptamers are nucleic acid molecules having specific binding affinity to molecules through interactions other than classic Watson-Crick base pairing.
Aptamers, like peptides generated by phage display or monoclonal antibodies (MAbs), are capable of specifically binding to selected targets and, through binding, block their targets' ability to function. Created by an in vitro selection process from pools of random sequence oligonucleotides (FIG. 1), aptamers have been generated for over 100 proteins including growth factors, transcription factors, enzymes, immunoglobulins, and receptors. A typical aptamer is 10-15 kDa in size (30-45 nucleotides), binds its target with sub-nanomolar affinity, and discriminates against closely related targets (e.g., will typically not bind other proteins from the same gene family). A series of structural studies have shown that aptamers are capable of using the same types of binding interactions (hydrogen bonding, electrostatic complementarity, hydrophobic contacts, steric exclusion, etc.) that drive affinity and specificity in antibody-antigen complexes.
Aptamers have a number of desirable characteristics for use as therapeutics (and diagnostics) including high specificity and affinity, biological efficacy, and excellent pharmacokinetic properties. In addition, they offer specific competitive advantages over antibodies and other protein biologics, for example:
1) Speed and Control.
Aptamers are produced by an entirely in vitro process, allowing for the rapid generation of initial (therapeutic) leads. In vitro selection allows the specificity and affinity of the aptamer to be tightly controlled and allows the generation of leads against both toxic and non-immunogenic targets.
2) Toxicity and Immunogenicity.
Aptamers as a class have demonstrated little or no toxicity or immunogenicity. In chronic dosing of rats or woodchucks with high levels of aptamer (10 mg/kg daily for 90 days), no toxicity is observed by any clinical, cellular, or biochemical measure. Whereas the efficacy of many monoclonal antibodies can be severely limited by immune response to antibodies themselves, it is extremely difficult to elicit antibodies to aptamers (most likely because aptamers cannot be presented by T-cells via the MHC and the immune response is generally trained not to recognize nucleic acid fragments).
3) Administration.
Whereas all currently approved antibody therapeutics are administered by intravenous infusion (typically over 2-4 hours), aptamers can be administered by subcutaneous injection. This difference is primarily due to the comparatively low solubility and thus large volumes necessary for most therapeutic MAbs. With good solubility (>150 mg/ml) and comparatively low molecular weight (aptamer: 10-50 kDa; antibody: 150 kDa), a weekly dose of aptamer may be delivered by injection in a volume of less than 0.5 ml. Aptamer bioavailability via subcutaneous administration is >80% in monkey studies (Tucker et al., J. Chromatography B. 732: 203-212, 1999). In addition, the small size of aptamers allows them to penetrate into areas of conformational constrictions that do not allow for antibodies or antibody fragments to penetrate, presenting yet another advantage of aptamer-based therapeutics or prophylaxis.
4) Scalability and Cost.
Therapeutic aptamers are chemically synthesized and consequently can be readily scaled as needed to meet production demand. Whereas difficulties in scaling production are currently limiting the availability of some biologics and the capital cost of a large-scale protein production plant is enormous, a single large-scale synthesizer can produce upwards of 100 kg oligonucleotide per year and requires a relatively modest initial investment. The current cost of goods for aptamer synthesis at the kilogram scale is estimated at $500/g, comparable to that for highly optimized antibodies. Continuing improvements in process development are expected to lower the cost of goods to <$100/g in five years.
5) Stability.
Therapeutic aptamers are chemically robust. They are intrinsically adapted to regain activity following exposure to heat, denaturants, etc. and can be stored for extended periods (>1 yr) at room temperature as lyophilized powders. In contrast, antibodies must be stored refrigerated.
Interstitial Fluid Pressure
The three most common types of cancer treatment are surgical removal of cancerous tissue, radiotherapy to obliterate cancerous tissue, and chemotherapy. These treatments are aimed at removing the cancerous tissues or cells or destroying them in the body with therapeutics or other agents. Chemotherapy remains a major treatment modality for solid tumors. To potentially reduce toxic side effects and to achieve higher efficacy of chemotherapeutic drugs, strategies to improve distribution of drugs between normal tissues and tumors are highly desirable.
A major obstacle in the treatment of solid tumors is the limited uptake of therapeutic agents into tumor tissue. Elevated interstitial fluid pressure (“IFP”) is one of the physiologically distinctive properties of solid tumors that differ from healthy connective tissue and is considered to be the main obstacle limiting free diffusion of therapeutics into solid tumors. PDGF receptors, particularly PDGF-β, have been implicated in the regulation of IFP. As a tumor enters a hyperproliferative state, blood supplying oxygen and other nutrients cannot keep up with the tumors' demands and a state of hypoxia results. Hypoxia triggers an “angiogenic switch” which will up-regulate the expression of several factors including VEGF and PDGF which in turn serve to initiate angiogenesis. However, the angiogenesis that results forms an abnormal tumor vasculature. The tumor vasculature becomes impaired to the point of being unable to adequately drain excess fluid from the interstitium and fluid accumulation distends the elastic interstitial matrix causing an increase in pressure. When pressure exceeds capillary wall resistance, compression occurs and blood flow resistance increases.
This property of most solid tumors—tumor interstitial hypertension or increased IFP—has been suggested as a potential target for efforts to increase tumor drug uptake (Jain et al., (1987) Cancer Res., 47:3039-3051). Increased IFP acts as a barrier for tumor transvascular transport (Jain et al. (1997), Adv. Drug Deliv. Rev. 26:71-90). Lessening of tumor IFP, or modulation of microvascular pressure, has been shown to increase transvascular transport of tumor-targeting antibodies or low-molecular weight tracer compounds (Pietras et al., (2001), Cancer Res., 61, 2929-2934). The etiology of interstitial hypertension in tumors is poorly understood. One proposed theory is that the lack of lymphatic vessels in tumors is a contributing factor to the increased tumor IFP (Jain et al., (1987), Cancer Res., 47:3039-3051). Another proposed theory is that the microvasculature and the supporting stroma compartment are likely to be important determinants for tumor IFP (Pietras et al., (2002) Cancer Res., 62: 5476-5484). Accumulating evidence points toward the transmembrane PDGF β-receptor tyrosine kinase as a potential target for pharmacological therapeutics to modulate tumor interstitial hypertension. Among other potential targets are growth factors that bind to the PDGF β-receptor.
PDGF Mediated Cancer
In addition to IFP and the difficulty of penetrating tumors with therapeutics, another obstacle in cancer treatment are mutations in certain forms of cancer by PDGF mediated cancer leading to constitutive expression of PDGF. These mutations drive abnormal proliferation of cells which results in the various forms of cancer as shown in FIG. 3 (Pietras et al., (2001), Cancer Res., 61, 2929-2934). A gene mutation results in amplification of PDGF α-receptors in high grade glioblastomas. In chronic myelomonocytic leukemia (CMML), constitutive activation of PDGF-β receptors results from a mutation which causes the fusion of β-receptors with proteins other than PDGF (Golub et al., (1994) Cell 77, 307-316, Magnusson et al., (2001) Blood 100, 623-626). Constitutive activation of PDGF-α-receptor due to activating point mutations has also been identified in patients with gastrointestinal stromal tumors (GIST) (Heinreich et al., (2003) Science 299, 708-710). Dermatofibrosarcoma protuberans (DFSP) is associated with constitutive production of fusion proteins which are processed to PDGF-BB (O'Brian et al., (1998) Gene Chrom. Cancer 23, 187-193; Shimiziu et al. (1999) Cancer Res. 59, 3719-3723; Simon et al. (1997) Nat. Genet., 15, 95-98). In addition to the constitutive activation of PDGF ligand and/or receptor due to mutations, up regulation has been shown in soft tissue sarcomas and gliomas (Ostman and Heldin, (2001), Adv. Cancer Res. 80, 1-38).
PDGF
Growth factors are substances that have a cell-proliferative effect on cells or tissues. Any given growth factor may have more than one receptor or induce cell proliferation in more than one cell line or tissue. PDGF belongs to the cysteine-knot growth factor family and was originally isolated from platelets for promoting cellular mitogenic and migratory activity. PDGF is a strong mitogen and has a pivotal role in regulation of normal cell proliferation such as fibroblasts, smooth muscle cells, neuroglial cells and connective-tissue cells. In addition, PDGF mediates pathological cell growth such as in proliferative disorders, and also plays a role in angiogenesis. Another growth factor involved in tumor angiogenesis is vascular endothelial growth factor (VEGF).
Four PDGF polypeptide chains have been identified which are currently known to make up five dimeric PDGF isoforms: PDGF-AA, -BB, -CC, -DD, and -AB. The most abundant species are PDGF AB and BB. PDGF isoforms bind to α and β tyrosine kinase receptors. PDGF receptors are expressed by many different cell types within tumors. The binding of PDGF isoforms to their cognate receptors induces the dimerization and subsequent phosphorylation of specific residues in the intracellular tyrosine kinase domain of the receptors and activation of the signaling pathway. PDGF isoforms -AA, -BB, -CC, and -AB induce PDGF α-receptor dimerization. PDGF-BB and PDGF-DD activate PDGF β receptor dimerization. All isoforms of PDGF except PDGF-AA activate both α and β receptors in cells which co-express both receptor types (FIG. 2). Because they are potent mitogens, PDGF isoforms have been targeted for proliferative disease therapeutics development, such as cancer, diabetic retinopathy, glomerulonephritis, and restenosis.
PDGF, which is secreted by endothelial cells, acts as direct mitogen for fibroblasts, recruits pericytes and stimulates vascular smooth muscle cells. Many solid tumors display paracrine signaling of PDGF in the tumor stroma. PDGF is known to up-regulate synthesis of collagen and to mediate interactions of anchor proteins such as integrins with extracellular matrix (ECM) components. PDGF interactions between connective tissue, ECM and intracellular actin filament systems cause increased tensile strength which contributes to high IFP. High IFP is localized to the site of tumor and is associated with poor prognosis in human cancers as it increases with tumor size and severity and the grade of malignancy. The role of PDGF signaling in control of IFP and the up-regulated expression in various solid tumors, has prompted investigation into whether the inhibition of PDGF signaling can decrease IFP and thereby increase drug uptake into solid tumors. Previous work has demonstrated that inhibition of PDGF signaling with small molecule receptor antagonists and a PDGF specific aptamer decreases interstitial fluid pressure and increases the uptake of chemotherapeutics into solid tumors (Pietras et al., (2001), Cancer Res., 61: 2929-2934).
Accordingly, it would be beneficial to have novel materials and methods in oncology therapy to reduce tumor IFP, decrease tumor angiogenesis, and reduce the deleterious effects of mutation by the constitutive expression of PDGF. The present invention provides materials and methods to meet these and other needs.